1. Field of the Invention
The present invention relates to a method of emplacing explosive charges in deep boreholes and to an apparatus for carrying out the method.
2. Description of the Prior Art
Processes for explosively fracturing deep rock are becoming increasingly important as it becomes necessary to tap deep mineralized rock masses in order to supplement or replace dwindling energy sources and minerals supplies. Deep-blasting processes are needed, for example, in the treatment of oil and gas wells, the preparation of metallic ores for in situ leaching, and the preparation of oil shale for retorting or solvent extraction in place. To carry out such processes it is sometimes necessary to emplace and detonate explosives in boreholes ranging in depth from seveal hundred to several thousand feet.
Conventional procedures for loading boreholes are not adequate for loading boreholes of such depth. For deep boreholes, the explosive needs to be largely in cartridged form to avoid such problems as desensitization of the explosive by water and leakage of the explosive through fissures in the surrounding rock. Because deep holes are apt to be flooded with water, explosives which do not have good water resistance must be enclosed in a water-resistant wrapper. Even water-bearing explosives, known as water gels, which have a relatively high degree of water resistance and often are used in bulk form, preferably are cartridged for deep borehole loading. If a relatively fluid water gel explosive is simply dumped down the borehole, much of it may never reach bottom because it may stick to the walls of the borehole or dissolve in water on the way down. That which does reach bottom may be desensitized by admixture with water, and some may subsequently leak away through fissures in the rock. Also, water gel explosives that are sufficiently fluid to be pumped rapidly through a hose to great depths are likely to have insufficient viscosity to resist desensitization by water, and may leak away through fissures in the rock.
On the other hand, if an attempt is made to dump down a borehole a water gel which has a sufficiently high viscosity to resist desensitization by water, the explosive may bridge across the hole and never reach bottom. Or, even if it reaches bottom, it may fail to slump into a high-density load because of its high viscosity. Although it is possible to formulate water gel explosives so that they are very fluid while being pumped down a hose to the depths of a deep hole and then develop a high viscosity after being emplaced, the precise chemical control required in such a system is difficult to achieve consistently under field conditions. The use of cartridged water gels eliminates the problems caused by too high or too low a viscosity in the bulk explosive.
When conventional borehole loading procedures are employed to emplace explosive cartridges in boreholes, i.e., dropping or lowering the cartridges from the borehole collar, the borehole may become bridged by one or more cartridges before they have reached bottom, thereby blocking the hole causing a pile-up of cartridges above it. Such an occurrence, which, at the very least, adds greatly to the borehole loading time, becomes even more likely with boreholes of great depth, and also more difficult to remedy.
In addition to the above-described difficulties, other problems are encountered in cases in which the borehole has been chambered to permit the emplacement of a larger charge or if the composition of the charge is to be varied as a function of either vertical position in the hole or hole diameter. In such cases it is important to know the height of rise of the explosive load as a continuous function of the load emplaced, in order to confirm the size and shape of the chamber. Also, in response to this variable, it is often desired that the composition of the explosive being loaded by quickly changed, or loading be quickly cut off entirely.
Also, because of the large number of cartridges which may need to be emplaced in the borehole, it is important from the standpoint of economics that the loading method be as rapid as possible.